More than 100 fungal species have been reported to harbor viruses (mycoviruses), (K. W. Buck, Fungal virology--an overview (CRC Press, Boca Raton, Fla., 1985)). Mycovirus infections are persistent, non-transmissible by an extracellular route and generally non-symptomatic, (Buck, supra and R. B. Wickner, FASEB 3, 2257 (1989)). However, members of the genus Hypovirus cause profound phenotypic alterations, including virulence attenuation (hypovirulence), in their host, the chestnut blight fungus Cryphonectria parasitica. Hypoviruses can be transmitted intercellularly to compatible virulent strains of C. parasitica through fused hyphae (anastomoses), (Van Alfen et al., P. R. Day, Science 189, 890 (1975) and S. L. Anagnostakis, P. R. Day, Phytopathology 69, 1226 (1979)). Thus, the application of hypovirus-attenuated C. parasitica strains can, under appropriate conditions, reduce or control progression of this fungal disease (Anagnostakis, S. L. (1982). Science, 215-466-471; S. L. Anagnostakis (1990), Forest Science 36, 113-124; Grente, J. and S. Berthelay-Sauret, (1978), p. 30-34. in W. L MacDonald, F. C. Cech, J. Luchok and H. C. Smith (eds) Proceedings of American Chestnut Symposium. West Virginia University Press, Morgantown; (Murr.). (Bart. S. L. Anagnostakis, Science 215, 466 (1982)).
The prototypic hypovirus, isolate CHV1-713 from hypovirulent C. parasitica strain EP713, is found predominantly as unencapsidated double-stranded RNA (L-dsRNA) with an organization similar to a replicative intermediate of a positive strand RNA virus, (Shapira, et al. EMBO J. 10, 731 (1991)). Attempts to artificially transmit hypoviruses by introduction of viral dsRNA into virus-free fungal strains have been unsuccessful (N. K. Van Alfen, Hypovirulence of Endothia (Cryphonectria) parasitica and Rhizoctonia solani. (CRC Press, Boca Raton, Fla., 1986)). Applicants have discovered that a hypovirus infection can be established in pathogenic fungi by introducing a synthetic copy of the L-dsRNA coding strand into fungal cells, resulting in reduced virulence.